How to verify if rect is inside cv::Mat in OpenCV? - c++

Question

Is there anything like cv::Mat::contains(cv::Rect) in Opencv?
Background:
After detecting objects as contours and trying to access ROIs by using cv::boundingRect my application crashed. OK, that's because the bounding rects of the object close to image border may be not entirely within the image.
Now I skip the objects not entirely in image by this check:
if(
cellRect.x>0 &&
cellRect.y>0 &&
cellRect.x + cellRect.width < m.cols &&
cellRect.x + cellRect.width < m.rows) ...
where cellRect is the bounding rect of the object and m is the image.
I hope there is a dedicated opencv function for this.

Answer 1

Simple way is to use the AND (i.e. &) operator.
Assume you want to check if cv::Rect rect is inside cv::Mat mat:
bool is_inside = (rect & cv::Rect(0, 0, mat.cols, mat.rows)) == rect;

Answer 2

You can create rect "representing"(x,y = 0, width and height equal to image width and height) your image and check whether it contains bounding rects of your contours. To achieve that you need to use rect intersection - in OpenCV it's very simple, just use rect1 & rect2. Hope that code makes it clear:
cv::Rect imgRect = cv::Rect(cv::Point(0,0), img.size());
cv::Rect objectBoundingRect = ....;
cv::Rect rectsIntersecion = imgRect & objectBoundingRect;
if (rectsIntersecion.area() == 0)
//object is completely outside image
else if (rectsIntersecion.area() == objectBoundingRect.area())
//whole object is inside image
else
//((double)rectsIntersecion.area())/((double)objectBoundingRect.area()) * 100.0 % of object is inside image

Answer 3

Here is a method to judge whether a rectangle contains an other rectangle.
you can get the size info from cv::Mat first, and then use method below:
public bool rectContainsRect(Rectangle containerRect, Rectangle subRect)
{
if( containerRect.Contains(new Point(subRect.Left, subRect.Top))
&& containerRect.Contains(new Point(subRect.Right, subRect.Top))
&& containerRect.Contains(new Point(subRect.Left, subRect.Bottom))
&& containerRect.Contains(new Point(subRect.Right, subRect.Bottom)))
{
return true;
}
return false;
}